From 548c2148d9be01ea3439c2d27aac49cd816ef084 Mon Sep 17 00:00:00 2001
From: Cody Maloney <cmaloney@theoreticalchaos.com>
Date: Wed, 17 Jun 2026 11:30:29 -0700
Subject: [PATCH] gh-71448: Support extension in BytesIO.truncate

Truncate beyond current size of I/O objects is documented to change the
amount of allocated space. BytesIO supported truncate to shrink but not
truncate to extend. Update BytesIO to support extension and fully
implement the `IOBase.truncate()` API.
---
 Doc/library/io.rst                            |  7 ++++++
 Lib/_pyio.py                                  |  2 +-
 Lib/test/test_io/test_memoryio.py             | 25 +++++++++++++++++++
 ...6-06-25-22-43-34.gh-issue-71448.ajvMxg.rst |  2 ++
 Modules/_io/bytesio.c                         | 11 ++++++--
 5 files changed, 44 insertions(+), 3 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-06-25-22-43-34.gh-issue-71448.ajvMxg.rst

diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index d47b74efe22de9d..ca35b97c754080b 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -785,6 +785,13 @@ than raw I/O does.
 
       .. versionadded:: 3.5
 
+   .. method:: truncate(size=None, /)
+
+      In :class:`BytesIO`, this is the same as :meth:`IOBase.truncate`.
+
+      .. versionchanged:: next
+         Now extends the underlying buffer as :meth:`IOBase.truncate` documents.
+
 .. class:: BufferedReader(raw, buffer_size=DEFAULT_BUFFER_SIZE)
 
    A buffered binary stream providing higher-level access to a readable, non
diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 4ba9b4070dff93e..4693993d9a13d8d 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -1016,7 +1016,7 @@ def truncate(self, pos=None):
                     pos = pos_index()
                 if pos < 0:
                     raise ValueError("negative truncate position %r" % (pos,))
-            del self._buffer[pos:]
+            self._buffer.resize(pos)
         return pos
 
     def readable(self):
diff --git a/Lib/test/test_io/test_memoryio.py b/Lib/test/test_io/test_memoryio.py
index 3669ac0b038b71b..e8b159fcc0d73ec 100644
--- a/Lib/test/test_io/test_memoryio.py
+++ b/Lib/test/test_io/test_memoryio.py
@@ -558,6 +558,31 @@ def test_relative_seek(self):
         memio.seek(1, 1)
         self.assertEqual(memio.read(), buf[1:])
 
+    def test_truncate_extend(self):
+        # gh-71448: Extending with truncate should allocate space.
+        buf = self.buftype("123")
+        memio = self.ioclass(buf)
+
+        self.assertEqual(memio.tell(), 0)
+        self.assertEqual(memio.truncate(4), 4)
+        self.assertEqual(len(memio.getbuffer()), 4)
+        self.assertEqual(memio.getvalue(), b"123\x00")
+        self.assertEqual(memio.tell(), 0)  # Truncate keeps pos.
+        # Truncate to position 0 should work.
+        self.assertEqual(memio.truncate(), 0)
+        self.assertEqual(memio.getvalue(), b"")
+
+        self.assertEqual(memio.seek(12), 12)
+        self.assertEqual(memio.truncate(1), 1)
+        self.assertEqual(len(memio.getbuffer()), 1)
+        self.assertEqual(memio.getvalue(), b"\x00")
+        self.assertEqual(memio.tell(), 12)
+
+        self.assertEqual(memio.truncate(), 12)
+        self.assertEqual(len(memio.getbuffer()), 12)
+        self.assertEqual(memio.getvalue(), b"\x00" * 12)
+        self.assertEqual(memio.tell(), 12)
+
     def test_issue141311(self):
         memio = self.ioclass()
         # Seek allows PY_SSIZE_T_MAX, read should handle that.
diff --git a/Misc/NEWS.d/next/Library/2026-06-25-22-43-34.gh-issue-71448.ajvMxg.rst b/Misc/NEWS.d/next/Library/2026-06-25-22-43-34.gh-issue-71448.ajvMxg.rst
new file mode 100644
index 000000000000000..05336eb85239471
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-06-25-22-43-34.gh-issue-71448.ajvMxg.rst
@@ -0,0 +1,2 @@
+Update :meth:`io.BytesIO.truncate` to match :meth:`io.IOBase.truncate`
+resize extension behavior. All resize behavior now matches.
diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index 8cdcbd0d89c718e..07c5fbfb6029868 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -667,10 +667,17 @@ _io_BytesIO_truncate_impl(bytesio *self, PyObject *size)
         }
     }
 
-    if (new_size < self->string_size) {
+    if (new_size != self->string_size) {
+        Py_ssize_t orig_size = self->string_size;
         self->string_size = new_size;
-        if (resize_buffer_lock_held(self, new_size) < 0)
+        if (resize_buffer_lock_held(self, new_size) < 0) {
             return NULL;
+        }
+        /* Fill new space with zeros */
+        if (new_size > orig_size) {
+            memset(PyBytes_AS_STRING(self->buf) + orig_size, '\0',
+                   (new_size - orig_size) * sizeof(char));
+        }
     }
 
     return PyLong_FromSsize_t(new_size);